Piping systems having secondary containment are required for the transport of hazardous fluid substances. Secondary containment minimizes the risk of damaging the environment in the event of leakage from the primary pipe and can assist in stabilizing conditions in the primary pipe. The leakage of hazardous fluids even in small amounts over long periods of time can produce a serious environmental hazard which can be difficult to detect and difficult and expensive to clean up once it has ultimately been detected.
Secondary containment systems for piping are used in many industries where the fluid transported in the primary pipe is environmentally hazardous or where there is concern for the consequences of pipe leakage. In a secondary containment system, a secondary containment vessel surrounds the primary pipe. One particular example of a secondary containment system is where the secondary containment vessel is a substantially coaxial or concentric pipe of greater diameter than the primary pipe. This is known as a double-walled piping system. Double-walled piping systems comprise a primary, inner pipe that carries a fluid and an outer annular space between the outer surface of the inner pipe and the inner surface of the secondary containment pipe. This annular space surrounds the inner pipe so as to capture and contain any leaking fluid that exits in the inner pipe.
One application of double-walled piping systems is in service station environments where fuel is delivered to fuel dispensers from underground storage tanks with coaxial secondary containment pipes. Another application of double-walled piping systems is in cryogenic liquid transport pipelines. These are used to move super cooled liquidized fluids such as liquid natural gas. In order to maintain the super cooled condition of a cryogenic fluid within a cryogenic liquid transport pipeline, the pipeline is constructed with a central axially extending mainline pipe which carries the super cooled liquidized fluid and a concentric secondary containment pipe evacuated to provide a vacuum insulation for the primary pipe.
Yet another example of a piping system having secondary containment is one used in nuclear applications, sometimes termed Active Drain Systems (ADS), where radioactive liquid wastes are transferred from various sources to a common point. Such drainage systems comprise a primary drain pipe with an additional secondary pipe to act as a containment pipe in the event of leakage from the primary pipe. These also typically have an open ended drain pipe that is not designed to hold significant pressure. These pipe systems can be buried underground. These systems can also be sloped in order to allow fluids to flow under gravity, and may not be designed to retain or sustain pressure.
In some systems, the ADS pipe length is hundreds of feet long and buried underground and, consequently, cannot be directly inspected. As the primary pipe of such systems is not pressure rated, performing a pressure decay test by applying vacuum or pressure to such a system is not an option. Accordingly, a method for detecting leakage in the primary pipe of such a double-walled pipe system, and/or determining the location of a leak in such a system is highly desirable.
Several methods are currently used for leak detection in containment systems. The simplest methods include visual inspection and exposure of the leak site to soap bubbles to determine the leak location. Another method is acoustic leak detection, which includes detection of the noise generated by escaping fluid using microphones to identify a leak location. This technique is typically used where there are relatively high flow rates and the system is pressurized with a liquid. This method has been applied to underground water supply piping systems.
Another common test to determine if a system contains a leak is a pressure or vacuum decay test, wherein a closed system is pressurized or pumped to a partial vacuum condition. The system will gradually lose pressure or vacuum if a leak is present. This particular test cannot be performed on systems that are not designed to sustain pressure.
Another leak detection method utilizes the principle of pulsed energy reflection, wherein a change in impedance in an electrical conductor installed in the secondary containment will cause a change in signal. The presence of such a signal can be interpreted as a leak, and can also be used to determine the location of the leak. However, the sensitivity of this method is sometimes insufficient and often requires a major leak before actuating an alarm. This system has also been shown to be susceptible to false-alarms caused by moisture in the system that is not the result of a leak.
A need remains for a system and method that can detect leaks in the primary pipe, as well as the location of the leak(s). Knowledge of the location of the leak can help direct maintenance and/or allow improved efficiency in the repair of the leak. This is particularly true in buried systems or difficult to reach areas.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.